Frequency hopping spread spectrum passive acoustic wave identification device

ABSTRACT

A system and method for interrogating a passive acoustic transponder, producing a transponder signal having characteristic set of signal perturbations in response to an interrogation signal, comprising a signal generator, producing an interrogation signal having a plurality of differing frequencies; a receiver, for receiving the transponder signal; a mixer, for mixing the transponder signal with a signal corresponding to the interrogation signal, to produce a mixed output; an integrator, integrating the mixed output to define an integrated phase-amplitude response of the received transponder signal; and an analyzer, receiving a plurality of integrated phase-amplitude responses corresponding to the plurality of differing frequencies, for determining the characteristic set of signal perturbations of the passive acoustic transponder.

The present application is a Continuation-in-Part of U.S. patentapplication Ser. No. 08/914,282, filed Aug. 18, 1997, the entirety ofwhich is expressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to method and apparatus for interrogatinga passive acoustic identification device (transponder) using a frequencyhopping spread spectrum interrogation signal, and more particularly to asystem and method for analyzing a passive acoustic wave identificationdevice response to a frequency and time discontinuous interrogationsignal.

BACKGROUND OF THE INVENTION

A known radio frequency passive acoustic transponder system producesindividualized responses to an interrogation signal. The code space forthese devices may be, for example, 2¹⁶ codes, or more, allowing a largenumber of transponders to be produced without code reuse. These devicesprovide a piezoelectric substrate on which an aluminum pattern isformed, for example by a typical microphotolithography process, with aminimum feature size of, for example, one micron.

The aforementioned transponder devices include a surface acoustic wavedevice, in which an identification code is provided as a characteristictime-domain delay pattern in a retransmitted signal, in a system whichgenerally requires that the signal emitted from an exciting antenna benon-stationary with respect to a signal received from the tag. Thisensures that the reflected signal pattern is distinguished from theemitted signal, and can be analyzed in a plurality of states. Thisanalysis reveals the various delay components within the device. In sucha device, received RF energy is transduced onto a piezoelectricsubstrate as an acoustic wave by means of an interdigital electrodesystem, from whence it travels through the substrate, interacting withreflecting, delay or resonant/frequency selective elements in the pathof the wave. A portion of the acoustic wave is ultimately received by aninterdigital electrode system, which may be the same or different thanthe launch transducer, and retransmitted. These devices do not require asemiconductor memory nor external electrical energy storage system,e.g., battery or capacitor, to operate. The propagation velocity of anacoustic wave in a surface acoustic wave device is slow as compared tothe free space propagation velocity of a radio wave. Thus, the time fortransmission between the radio frequency interrogation system and thetransponder is typically short as compared to the acoustic delay of thesubstrate. This allows the rate of the interrogation frequency change tobe based primarily on the delay characteristics within the transponder,without requiring measurements of the distance between the transponderand the interrogation system antenna.

The interrogation frequency is controlled to change sufficiently fromthe return or “backscatter” signal from the transponder, so that areturn signal having a minimum delay may be distinguished from theinterrogation frequency, and so that all of the relevant delays areunambiguously received for analysis. The interrogation frequency thusshould not return to the same frequency within a minimum time-period.Generally, such systems are interrogated with a pulse transmitter orchirp frequency system.

Systems for interrogating a passive transponder employing acoustic wavedevices, carrying amplitude and/or phase-encoded information aredisclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160;4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208;4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790;4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein byreference. Other passive interrogator label systems are disclosed in theU.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217, expresslyincorporated herein by reference.

Passive transponder tag interrogation systems are also known withseparate receiving and transmitting antennas, which may be at the samefrequency or harmonically related, and having the same or differentpolarization. Thus, in these systems, the transmitted and receivedsignals may be distinguished other than by frequency. The acoustic waveis often a surface acoustic wave, although acoustic wave devicesoperating with various other wave types, such as bulk waves, are known.

The information code associated with and which identifies the passivetransponder is built into the transponder at the time that a layer ofmetallization is finally defined on the substrate of piezoelectricmaterial. This metallization also defines the antenna coupling, launchtransducers, acoustic pathways and information code elements, e.g.,reflectors and delay elements. Thus, the information code in this caseis non-volatile and permanent. The information representing theseelements is present in the return signal as a set of characteristicperturbations of the interrogation signal, such as a specific complexdelay pattern and attenuation characteristics. In the case of atransponder tag having launch transducers and a variable pattern ofreflective elements, the number of possible codes is N×2^(M) where N isthe number of acoustic waves launched by the transducers and M is thenumber of reflective element positions for each transducer. Thus, withfour launch transducers each emitting two acoustic waves, and apotential set of eight variable reflective elements in each acousticpath, the number of differently coded transducers is 2048. Therefore,for a large number of potential codes, it is necessary to provide alarge number of launch transducers and/or a large number of reflectiveelements. However, efficiency is lost with increasing acoustic pathcomplexity, and a large number of distinct acoustic waves reduces thesignal strength of the signal encoding the information in each.Therefore, the transponder design is a tradeoff between device codespacecomplexity and efficiency.

The known passive acoustic transponder tag thus typically includes amultiplicity of “signal conditioning elements”, i.e., delay elements,reflectors, and/or amplitude modulators, which are coupled to receivethe first signal from a transponder antenna. Each signal conditioningelement provides an intermediate signal having a known delay and a knownamplitude modification to the acoustic wave interacting with it. Evenwhere the signal is split into multiple portions, it is advantageous toreradiate the signal through a single antenna. Therefore, a “signalcombining element” coupled to the all of the acoustic waves, which haveinteracted with the signal conditioning elements, is provided forcombining the intermediate signals to produce the radiated transpondersignal. The radiated signal is thus a complex composite of all of thesignal modifications, which may occur within the transponder, modulatedon the interrogation wave.

In known passive acoustic transponder systems, the transponder remainsstatic over time, so that the encoded information is retrieved by asingle interrogation cycle, representing the state of the tag, or moretypically, obtained as an inherent signature of an emitted signal due tointernal time delays. In order to determine a transfer function of apassive transponder device, the interrogation cycle may includemeasurements of excitation of the transponder at a number of differentfrequencies. This technique allows a frequency domain analysis, ratherthan a time domain analysis of an impulse response of the transponder.This is particularly important since time domain analysis requires veryhigh time domain resolution, e.g., <100 nS, to accurately capture thecharacteristics of the encoding, while frequency domain analysis doesnot impose such stringent requirements on the analysis system.

Passive transponder encoding schemes include selective modification ofinterrogation signal transfer function H(s) and delay functions f(z).These functions therefore typically generate a return signal in the sameband as the interrogation signal. Since the return signal is mixed withthe interrogation signal, the difference between the two will generallydefine the information signal, along with possible interference andnoise. By controlling the rate of change of the interrogation signalfrequency with respect to a maximum round trip propagation delay,including internal delay, as well as possible Doppler shift, the maximumbandwidth of the demodulated signal may be controlled. Thus, the knownsystems seek to employ a chirp interrogation waveform which allows arelatively simple processing of limited bandwidth signals.

Typically, the interrogator transmits a first signal having a firstfrequency that successively assumes a plurality of incremental frequencyvalues within a prescribed frequency range. This first frequency may,for example, be in the range of 905-925 MHz, referred to herein as thenominal 915 MHz band, a frequency band that is commonly available forunlicensed use. Of course, other bands may be used, and preferably theseare bands which do not require a license, and are available worldwidefor use. Of course, licensed bands and locally available bands may beused. The response of the tag to excitation at any given frequency isdistinguishable from the response at other frequencies, due to therelationship of the particular frequency and fixed time delays.

Preferably, the passive acoustic wave transponder tag includes at leastone element having predetermined characteristics, which assists insynchronizing the receiver and allowing for temperature compensation ofthe system. As the temperature changes, the piezoelectric substrate mayexpand and contract, altering the characteristic delays and otherparameters of the tag. Variations in the transponder response due tochanges in temperature thus result, in part, from the thermal expansionof the substrate material. Although propagation distances are small, anincrease in temperature of only 20° C. can produce an increase inpropagation time by the period of one entire cycle at a transponderfrequency of about 915 MHz; correspondingly, a change of about 1° C.results in a relative phase change of about 18°.

A known transponder is constructed such that i^(th) delay timet_(i)=T₀+KΔT+ΔV_(i), where K is a proportionality constant, ΔT is thenominal, known difference in delay time between the intermediate signalsof two particular successive ones of the signal delay elements in thegroup, and ΔV_(i) is a modification factor due to inter-transpondervariations, such as manufacturing variations. By measuring thequantities ΔT and ΔV_(i), it is possible, according to known techniques,to determine the expected delay time t_(i)−T₀ for each and every signaldelay element from the known quantities K, ΔT and ΔV_(i). Themanufacturing variations ΔV_(i) comprise a “mask” variation ΔM_(i) dueto imperfections in the photolithographic mask; an “offset” variationΔO_(i) which arises from the manufacturing process used to deposit themetal layer on the piezoelectric substrate; and a random variationΔR_(i) which is completely unpredictable but usually neglectably small.Specific techniques are available for determining and compensating boththe mask variations ΔM_(i) and the offset variations ΔO_(i).

The known chirp interrogation system for interrogating surface acousticwave transponder system provides a number of advantages, including highsignal-to-noise performance. Further, the output of the signalmixer—namely, the signal which contains the instantaneous differencefrequencies of the interrogating chirp signal and the transponder replysignal, typically fall in the range below 3000 Hz, and thus may betransmitted over inexpensive, shielded, audio-grade twisted-pair wires.Furthermore, since signals of this type are not greatly attenuated ordispersed when transmitted over long distances, the signal processor maybe located at a position quite remote from the signal mixer, or providedas a central processing site for multiple interrogator antennae.

Another known type of interrogation system employs impulse excitation.These systems require broadband transponder signal analysis, and thuscannot typically employ audio frequency analysis systems. This impulseexcitation interrogation system does not seek to analyze the response offixed elements within the passive transponder to a plurality ofdifferent excitation challenges.

A known surface acoustic wave passive interrogator label system, asdescribed, for example, in U.S. Pat. Nos. 4,734,698; 4,737,790;4,703,327; and 4,951,057, expressly incorporated herein by reference,includes an interrogator having a voltage controlled oscillator whichproduces a first radio frequency signal determined by a control voltage.This first signal is amplified by a power amplifier and applied to anantenna for transmission to a remote transponder. As is known, thevoltage controlled oscillator may be replaced with other oscillatortypes.

The first signal is received by an antenna of the remote transponder andpassed to a signal transforming element, which converts the first(interrogation) signal into a second (reply) signal, encoded with acharacterizing information pattern. The information pattern is encodedby a series of elements having characteristic delay periods T₀ and ΔT₁,ΔT₂, . . . ΔT_(N).

Two common types of systems exist. In a first, the delay periodscorrespond to physical delays in the propagation of the acoustic signal.After passing each successive delay, a portion of the signal I₀, I₁, I₂. . . I_(N) is tapped off and supplied to a summing element. Theresulting signal S2, which is the sum of the intermediate signals I₀ . .. I_(N), is fed back to a transponder tag antenna, which may be the sameor different than the antenna which received the interrogation signal,for transmission to the interrogator/receiver antenna. In a secondsystem, the return signal is composed of sets of reflected signals,resulting from reflectors in the path of the signal which reflectportions of the acoustic wave back to the launch transducer, where theyare converted back to an electrical signal and emitted by thetransponder tag antenna. The second signal is passed either to the sameor different antenna of the remote transponder for transmission back tothe interrogator/receiver apparatus. In both cases, between the taps orreflectors, signal modification elements, such as delay pads,selectively modify the signal. This second signal carries encodedinformation which, at a minimum, identifies the particular transponder.

The transponder serves as a signal transforming element, which comprisesN+1 signal conditioning elements and a signal combining element. Thesignal conditioning elements are selectively provided to impart adifferent response code for different transponders, and which mayinvolve separate intermediate signals I₀, I₁ . . . I_(N) within thetransponder. Each signal conditioning element comprises a known delayT_(i) and a known amplitude modification A_(i) (either attenuation oramplification). The respective delay T_(i) and amplitude modificationA_(i) may be functions of the frequency of the received first signal,may provide a constant delay and constant amplitude modification,respectively, independent of frequency or may have differing dependencyon frequency. The order of the delay and amplitude modification elementsmay be reversed; that is, the amplitude modification elements A_(i) mayprecede the delay elements T_(i). Amplitude modification A_(i) can alsooccur within the path T_(i). The signals are combined in a combiningelement which combines these intermediate signals (e.g., by addition,multiplication or the like) to form the second (reply) signal S2 and thecombined signal is emitted by the transponder antenna.

The second signal is picked up by a receiving antenna of theinterrogation apparatus. Both this second signal and the first signal(or respective signals derived from these two signals) are applied to amixer (four quadrant multiplier) to produce a third signal containingfrequencies which include both the sums and the differences of thefrequencies contained in the first and second signals. The third signalis then low-pass filtered, digitized and passed to a digital signalprocessor which determines the amplitude a_(i) and the respective phaseφ_(i) of each frequency component f_(i) among a set of frequencycomponents (f₀, f₁, f₂ . . . ) in the filtered third signal. The filterthus distinguishes the sum and difference components, and preventsaliasing in the analog-to-digital converter. Typically, the low passfilter is set to have a narrow passband, to filter transients and reduceGaussian noise. For example, in a known system with a frequency hoppingrate of 8,000 per second, the filter has a cutoff of about 3,000 Hz.This narrow bandwidth allows a relatively slow analog to digitalconverter, e.g., about 10 ksps, to be employed to digitize the signal.

Each phase φ_(i) is determined with respect to the phase φ₀=0 of thelowest frequency component f₀. The third signal may be intermittentlysupplied to the mixer by means of a switch, and indeed the signalprocessor may be time-division multiplexed to handle a plurality ofmixed (demodulated) signals from different antennas.

The information determined by the digital signal processor is passed toa microprocessor computer system. This computer system analyzes thefrequency, amplitude and phase information and makes decisions basedupon this information. For example, the computer system may determinethe identification number of the interrogated transponder. This I.D.number and/or other decoded information is made available at an output.

In one known interrogation system embodiment, the voltage controlledoscillator is controlled to produce a sinusoidal RF signal with afrequency that is incrementally swept in 128 equal discrete steps from905 MHz to 925 MHz. Each frequency step is maintained for a period of125 microseconds so that the entire frequency sweep is carried out in 16milliseconds, with a step rate of 8 kHz. Thereafter, the frequency isdropped back to 905 MHz in a relaxation period of 0.67 milliseconds.This stepwise frequency sweep approximates a linear frequency sweep. Inthis embodiment, each delayed component within the reply (second) signalhas a different frequency with respect to the instantaneousinterrogation (first) signal.

Assuming a round-trip, radiation transmission time of t₀, the totalround-trip times between the moment of transmission of the first signaland the moments of reply of the second signal will be to t₀+T₀, t₀+T₁, .. . t₀+T_(N), for the delays T_(0N), T . . . , T₁ respectively.Considering only the transponder delay T_(N), at the time t_(R) when thesecond (reply) signal is received at the antenna, the frequency of thissecond signal will be Δf_(N) less than the instantaneous frequency ofthe first signal transmitted by the interrogator system antenna. Thus,if the first and second signals are mixed or “homodyned”, this frequencydifference Δf_(N) will appear in the third signal as a beat frequency.Understandably, other beat frequencies will also result from the otherdelayed frequency spectra resulting from the time delays T₀, T₁ . . .T_(N−1). In the case of a “chirp” waveform, the difference between theemitted and received waveform will generally be constant, and thereforethe relationship of each delayed component can be determined.

In mathematical terms, we assume that the phase of a transmittedinterrogation signal is φ=2πfτ, where τ is the round-trip transmissiontime delay. For a ramped frequency df/dt or f, we have: 2πfτ=dφ/dt=ω. ω,the beat frequency, is thus determined by τ for a given ramped frequencyor chirp f. In this case, the third (mixed) signal may be analyzed bydetermining a frequency content of the third signal, for example byapplying it to sixteen bandpass filters, each tuned to a differentfrequency, f₀, f₁ . . . f_(E), f_(F). The signal processor determinesthe amplitude and phase of the signals that pass through theserespective filters. These amplitudes and phases contain the code or“signature” of the particular signal transformer of the interrogatedtransponder. This signature may be analyzed and decoded in known manner.

As can be seen, in this embodiment, all significant components of thethird (mixed) signal will be within a limited range defined by themaximum delay within the transponder signal transformer and the chirprate. Thus, this signal may be band limited within this range withoutloss of significant information. As stated above, in a known system witha chirp range of 20 MHz, over a cycle period of 16 mS, with 128transitions, the frequency difference per transition is 156,250 Hz.

In one embodiment of a passive transponder, the internal circuit is asurface acoustic wave device which operates to convert the receivedfirst signal into an acoustic wave, and then to reconvert the acousticenergy back into the second signal for transmission via a dipoleantenna. The signal transforming element of the transponder includes apiezoelectric substrate material such as a lithium niobate (LiNbO₃)crystal, which has a free surface acoustic wave propagation velocity ofabout 3488 meters/second. The substrate is, for example, a 3 mm by 5 mmrectangular slab having a thickness of 0.5 mm. On the surface of thissubstrate is deposited a layer of metal, such as aluminum, forming apattern which includes transducers and delay/reflective elements. Eachdelay element has a width sufficient to delay the propagation of thesurface acoustic wave from one tap transducer to the next by one quartercycle or 90° with respect to an undelayed wave at the frequency ofoperation (in the 915 MHz band). By providing locations for three delaypads between successive tap transducers, the phase f of the surfaceacoustic wave received by a tap transducer may be controlled to providefour phase possibilities, zero pads=0°; one pad=90°; two pads=180°; andthree pads=270°. These pads may be selectively deposited as ametallization layer during manufacture, or formed in a completecomplement and selectively removed during a secondary process to encodethe transponder. Where a reflective element returns the signal to thelaunch transducer, the delays are calculated based on two passes overthe pad. Typically, a reflective or semireflective element is providedbetween each set of delay pads to allow them to be distinguished, andallowing, in the case of semireflective elements, for a series of setsof delay pads to be disposed along the path of an acoustic wave. As thenumber of sets of delay pads increases, the signal to noise ratio in thetransponder reply signal is severely degraded. This limitation on thenumber of tap transducers places a limitation on the length of theinformational code imparted in the transponder replies.

A plurality of launch transducers may be connected to common bus barswhich, in turn, are connected to the dipole antenna of the transponder.Each launch transducer may have a forward and backward wave, and,indeed, care must be taken to damp a reverse wave where this emission isundesired in order to reduce interference. Thus, the codespace of thetransponder may include a plurality of sets of encoding elements, eachassociated with a particular wavepath. Opposite each launch transduceris one or more reflectors, which reflect surface acoustic waves backtoward the transducers which launched them. Since the transducers areconnected in parallel, a radio frequency interrogation pulse is receivedby all the transducers essentially simultaneously. Consequently, thesetransducers simultaneously generate surface acoustic waves which aretransmitted outward in both directions. The system is configured so thatthe reflected surface acoustic waves are received by their respectivetransducers at staggered intervals, so that a single interrogation pulseproduces a series of reply pulses after respective periods of delay.

The embodiment of FIG. 1 comprises a substrate 120 of piezoelectricmaterial, such as lithium niobate, on which is deposited a pattern ofmetallization essentially as shown. The metallization includes two busbars 122 and 124 for the transmission of electrical energy to fourlaunch transducers 126, 128, 230 and 232. These launch transducers arestaggered, with respect to each other, with their leading edgesseparated by distances X, Y and Z, respectively, as shown. The distancesX and Z are identical; however, the distance Y is larger than X and Z inorder to provide temporal separation of the received signalscorresponding to the respective signal paths. Further metallizationincludes four parallel rows of delay pads 134, 136, 138 and 140 and fourparallel rows of reflectors 142, 144, 146 and 148. The two rows ofreflectors 144 and 146 which are closest to the transducers are calledthe “front rows” whereas the more distant rows 142 and 148 are calledthe “back rows” of the transponder. The bus bars 122 and 124 includecontact pads 150 and 152, respectively, to which are connected theassociated poles 152 and 156 of a dipole antenna. These two poles areconnected to the contact pads by contact elements or wires 158 and 160,represented in dashed lines.

The provision of four transducers 126, 128, 130 and 132 and two rows ofreflectors 142, 144, 146, and 148 on each side of the transducersresults in a total of sixteen SAW pathways of different lengths and,therefore, sixteen “taps”. These sixteen pathways (taps) are numbered 0,1, 2 . . . D, E, F, as indicated by the reference number (letter)associated with the individual reflectors. Thus, pathway 0 extends fromtransducer 226 to reflector 0 and back again to transducer 126. Pathway1 extends from transducer 228 to reflector 1 and back again totransducer 128. The spatial difference in length between pathway 0 andpathway 1 is twice the distance X (the offset distance betweentransducers 126 and 128). This results in a temporal difference of ΔT inthe propagation time of surface acoustic waves. Similarly, pathway 2extends from transducer 126 to reflector 2 and back again to transducer126. Pathway 3 extends from transducer 128 to reflector 3 and back totransducer 128. The distance X is chosen such that the temporaldifferences in the length of the pathway 2 with respect to that ofpathway 1, and the length of the pathway 3 with respect to that ofpathway 2 are also both equal to ΔT. The remaining pathways 4, 5, 6, 7 .. . E, D, F are defined by the distances from the respective transducerslaunching the surface acoustic waves to the associated reflectors andback again. The distance Y is equal to substantially three times thedistance X so that the differences in propagation times between pathway3 and pathway 4 on one side of the device, and pathway B and pathway Con the opposite side are both equal to ΔT. With one exception, all ofthe temporal differences, from one pathway to the next successivepathway are equal to the same ΔT. The SAW device is dimensioned so thatΔT nominally equals 100 nanoseconds. In order to avoid the possibilitythat multiple back and forth propagations along a shorter pathway (oneof the pathways on the left side of the SAW device as seen in FIG. 1)appear as a single back and forth propagation along a longer pathway (onthe right side of the device), the difference in propagation times alongpathways 7 and 8 is made nominally equal to 150 nanoseconds.

SUMMARY AND OBJECTS OF THE INVENTION

According to one embodiment of the present invention, the chirp(successive incrementally varying) interrogation signal of the prior artis replaced by a frequency hopping spread spectrum signal.

According to another embodiment, a frequency hopping interrogationsignal is pulse modulated, to have time discontinuities, so that aninterrogation signal is substantially not present when a transpondersignal is being received. In this case, the transponder response signalalso represents a frequency hopping signal, having a bandwidth of aboutthe entire width of the interrogation band, e.g., about 20 MHz, butdelayed from the respective interrogation signal.

The present invention also provides a system for analyzing spreadspectrum passive transponder signals. The signal analysis compensatesfor the “out of order” excitation sequence as compared to a chirpwaveform. According to another aspect of the present invention, thefrequency hopping sequence is demodulated to baseband, and the randomexcitation order accounted for.

The interrogation signal preferably has a gated 50% duty cycle. Theresult of this intermittent transmission is the elimination of thenear-field reflection of the interrogation signal. In addition, thetransmitter is decoupled during receipt of the transponder signal,resulting in improved impedance matching. Further, a direct coupling ofthe interrogation signal into the received signal is not observed.

Prior frequency hopping spread spectrum interrogation systems haveemployed continuous signals, which do not afford the present advantages.This is because the continuous frequency hopping spread spectrumtechnique allows continuous signal acquisition, and thus potentiallyfaster readings from a transponder tag.

The preferred intermittent sequence includes a transmit interrogationpulse of 7.5 μS, followed by a gap of about 300 nS, followed by alistening period of 4 μS. Thereafter, for about 3.5 μS, there is nosystem activity.

The present invention also provides a wide bandwidth detector, whichdetects the phase transitions of the signal resulting from mixing thereceived signal with a local oscillator signal. In this case, it iseasier to recover from noise in the filter or from a “bad hop”, whichcould saturate the detector for a relatively long period.

Prior art detector circuits, for a 915 MHz band system with 128 evenlyspaced hops of duration 125 μS, and exciting a transponder having amaximum internal delay of less than 10 μS, have a bandwidth of about 3kHz, with a frequency transition rate (hop rate) of 8 kHz. 3 kHz isabove the maximum significant frequency expected in the mixed signalrepresenting the instantaneous transmitted and return signal with achirp waveform interrogation signal under these conditions.

According to the present invention, a detector having a bandwidth ofabout 150 kHz is employed, in a system having a frequency hopping rateof about 66 kHz (hop period of about 15 μS). This wide bandwidthdetector provides fast recovery, low inter-symbol interference, andallows increased sample rates over prior methods.

Accordingly, it is seen that, in prior art systems, the bandwidth of thedetector was lower than the frequency hopping rate, while according tothis aspect of the present invention, the bandwidth of the detector isgreater than the frequency hopping rate. This system provides greaterimmunity from interference from continuous wave interference sourcesthan prior systems. In other words, whereas prior systems would lose,for example, data from about 30-50 frequency bins due to a continuouswave interference source, the wideband detector system would lose only 1or 2, a significant improvement.

Prior chirp based systems were also sensitive to the Doppler effect,such that a relative speed of about 50 mph would result in a bin shiftand potentially erroneous data. A preferred embodiment of the presentinvention provides a frequency hopping spread spectrum interrogationsystem which degenerates the Doppler effect by providing a “V chirp”,effectively a non-asymmetric frequency pattern, having an effectivedf/dt≈0, which cancels the Doppler shift effect and reduces these typesof errors.

As a result of the wide bandwidth detector, the signal coming into theanalog-to-digital converter is not narrow band limited, and thus theprocessing system must also accept a wideband signal. This also differsfrom prior practice, wherein, the significant analog signal was, forexample, presumed to be limited to a bandwidth of about 3 kHz.Therefore, in prior systems, with a narrow band low pass filter placedin the analog signal processing path, an analog to digital converterappropriate for this band-limited signal was employed, e.g., a 10 kspsconverter for a 3 kHz band-limited signal. According to the presentinvention, a high speed analog-to-digital converter is employed, forexample one capable of sampling the detected signal above its Nyquistrate, for example capable of accepting a signal having a 150 kHzbandwidth; for example a 300-500 ksps converter, preferably with trackand hold circuitry. Appropriate antialiasing circuits are employed forthe converter sample rate and desired bandwidth.

This wide bandwidth digitized signal thus presents a substantial amountof data to be analyzed. However, because a digital signal processingsystem is provided, an early data decimation scheme may be implemented,as can an adaptive algorithm and/or non-linear filtering to “clean up”the signal before extraction of the significant data. For example, inthe case of radio frequency interference from a fixed narrow bandsource, data samples corrupted by such interference can be eliminated,or even particularly analyzed in light of the interference, withoutdisrupting data acquisition from other frequency channels. As usedherein, a “channel” refers to a particular nominal frequency within theband of the interrogation signal. Typically, it is not necessary toanalyze data from each frequency bin to obtain a valid identification,as an excess of frequency bins may be provided; however the availabilityof such analytic capability potentially improves the robustness of thesystem and facilitates the use of multiple unsynchronized interrogationsystems in close proximity.

It is noted that, according to known systems, this band-limiting filter(detector) acted as an integrator for detecting the phase-amplituderesponse; the result of the modification of a given frequency excitationof the transponder tag is a received signal having a characteristicshift in phase and change in amplitude, which was seen, for example, asa beat frequency pattern in the return signal. While the phase shift andamplitude attenuation occurs in discrete portions, various known systemsseek only to read the net changes between the initial return signal fromthe transponder and the maximally delayed signal, after all internalelements were accounted for. Thus, it was desired to filter the signalto eliminate transients and increase signal to noise ratio by providinga long time-constant integration (time-averaging) of the signal.Further, since the signal to be analyzed was a mixed signal, a low passfilter also served as an image reject filter.

In contrast, while the system according to the present invention mayalso seek to determine the net phase and amplitude shift, the filter isnot used, for example, to eliminate transients in the response signal.This function is deferred to the digital signal processing chain.

The actual bandwidth of the receiver circuitry preferably is greaterthan the frequency hop rate in a spread spectrum or staircase incrementfrequency embodiment. Thus, with frequency changes every 15 μS, thepreferred filter has a minimum bandwidth of about 50-60 kHz. Of course,as the bandwidth of the filter is narrowed, the advantages of theinvention decrease, and likewise, the performance improves as thebandwidth gets to be large as compared to the frequency hop rate orchange rate.

It is noted that the frequency change rate is established so that thereturning signal from transponder tag differs from a simultaneouslytransmitted interrogation signal. In a pulse modulated embodiment, it issufficient if the simultaneously transmitted interrogation signal iszero Hertz or turned off. This allows improved impedance matching in thereceiver and reduced leakage from a simultaneously transmitted signal.The return signal timing is, in turn, based on the inherent delay in thetransponder tag and the time of flight travel through the atmosphere.Due to the relatively slow propagation of the signal as an acoustic wavethrough the transponder tag substrate, this time delay predominates.Thus, the frequency hop rate is normally set with a longer time constantthan the maximum significant delay within the transponder tag with areasonable margin. For example, if the transponder has a maximum delayperiod of less than 10 μS, then the frequency hop rate may be about 100kilohops per second. Practically, the interrogation system operates at aslower rate, for example 15 μS per hop. Therefore, in this case, theband-limiting filter will preferably have a minimum cutoff of 66-100kHz, depending on the particular interrogation system operationalparameters. Practically, a cutoff of 100-500 kHz is sufficient forachieving the benefits of the invention with a 15 μS frequency hop rate,with the preferred filter being 150 kHz under these interrogationparameters. Restated, the minimum preferred ratio of cutoff frequency tohop frequency is 1, with benefits significantly diminishing below 0.5,while a ratio of 1.8 to 8 is typically sufficient. Related to thereciprocal of the transponder delay, the preferred minimum cutoff is0.66, with a preferred range of 1.2-5.

It is noted that the received signal from the transponder tag hastransient signal components which have a minimum timing of 100 nS.Typically, the preferred interrogation system does not attempt todirectly read or interpret these transients, which would require, forexample, a 20 MHz or higher receiving system bandwidth, employing, forexample, a 20-50 MSPS analog to digital converter and producing a verylarge amount of raw data. However, such a sampling and analysistechnique is possible, for example, employing systems similar to radartarget analyzers.

The frequency hopping interrogation signal is generated by a digitallycontrolled oscillator to produce a pseudorandom pattern of frequencies.The digitally controlled oscillator is preferably a voltage controlledoscillator with a digital control input. The duration of each hop islonger than the longest delay in a transponder as well as the traveldelay. The duration of the interval between hops is also longer than thelongest delay in a transponder as well as the travel delay. Indetermining the transponder tag code, the transponder is excited by atleast the same number of differing hops as there are delay taps (degreesof freedom), and preferably are a larger number. For example, with 16delay taps, there are at least 16 excitation frequencies, and morepreferably 128 frequencies, which allows improved robustness ofoperation. This robustness provides immunity to random noise, fixedfrequency interference sources and adjacent interrogation systems.Advantageously, the pseudorandom sequence of the entire sequence offrequencies includes suitable subsets of excitation frequencies,allowing preliminary processing to determine transponder tag code tocommence prior to completion of an entire sequence. If the code isreliably read, the interrogation sequence may be truncated, for exampleto reduce power consumption in a portable embodiment or to reduce thepossibility of adjacent channel interference where multipleinterrogation systems are in close proximity. In this case, theinterrogation cycle may be discontinuous. The pseudorandom sequence mayrepeat after each set, or have extended pseudorandom properties.

The properties of the interrogation transmitter preferably spread thepower of the interrogation wave relatively evenly across the band overtime, to reduce interference with neighboring devices, and topotentially avoid functional interference from neighboring devices,which may be other interrogation systems, or other radio frequencysensitive devices. Thus, since the pseudorandom sequence includes a setof excitation frequencies larger than is needed, interference on one ormore particular excitation frequencies may be tolerated. Governmentregulatory agencies, such as the U.S. Federal Communications Commission(FCC) provide rules, regulations or guidelines as to what types offrequency hopping emissions are acceptable, and the system is capable ofoperating within the requirements presently established. The presentsystem, therefore, accommodates the various permissible schemes, and maybe modified according to the teachings herein to accommodate newlyproposed or required radio frequency emission characteristics.

Since the acoustic wave transponder device is passive, the order ofexcitation frequencies is accounted for in the analysis, to analyze areceived signal based on the corresponding excitation frequency. Sincethe signals are linearly additive and the filters may be highlyselective, multiple frequency channels may be present simultaneously,although a multichannel decoder (or multiple repetitions of a channelsequence) would be required to analyze the signal. Known acoustic wavetransponder devices may be used with the system according to the presentinvention, as the interrogation and signal analysis scheme are tolerantof various transponder designs.

In a preferred embodiment, the interrogation signal is pulsatile,having, for example, between 25%-50% duty cycle, and thereforerepresents a substantially discontinuous waveform. The prior art lowbandwidth filter/integrator would not appropriately settle under suchcircumstances. However, according to the present invention, thefilter/integrator has a short time constant relative to the frequencyhopping rate, and therefore a usable output is produced. For example,with a frequency hopping period of 15 μS, the system generates aninterrogation pulse at a selected frequency for 7.5 μS, is silent andinsensitive for 300 nS to eliminate near-field echos, and then listensfor 4 μS for the return signal. The system is then silent andinsensitive for the remaining portion of the cycle, about 3.5 μS.

The receiver mixes the interrogation signal, or a representationthereof, e.g., a delayed representation, with the received transpondersignal, which results in a signal modulated around baseband, with aseries of transients representing the taps of the transponder tag. Thenet phase shift and amplitude change after these transients settlerepresents the significant data from the tag, the superposition of eachsignal path. By measuring this superposed response for multiplefrequencies, the individual signal modification elements may be decoded.

Since the transponder signal is demodulated to baseband, the signal fromthe homodyne demodulator includes a near-D.C. component as well asbursts of high frequency energy corresponding to the transients producedby the delay taps, after each frequency hop. According to prior methods,the demodulator output is low pass filtered, for example less than about10 kHz, e.g., 3 kHz, to filter the bursts and high frequency componentsfrom the low frequency components representing the significant data.

In a homodyne embodiment, this low pass detector filter thus acts as anintegrator, whose output represents a near-D.C. voltage due to the phaseamplitude response, at a given excitation frequency, to a particular setof encoding elements of a transponder, i.e., phase shift elements andattenuation elements. Since, in the preferred embodiment, theattenuation elements are frequency independent, the normalized near-D.C.voltage signal will vary in proportion to the net phase shift from allof the signal modification elements, over short periods. Even where themixed signal is somewhat displaced from 0 Hz, such as due to a Dopplereffect, the detected output may be analyzed to determine the delayswithin the transponder.

This change in amplitude is determined for every frequency hop, forexample every 125 μS for an 8 kilohop per second embodiment, and every15 μS for a 66 kilohop per second embodiment. Because of the differencesin the transponder signal with respect to changes in interrogationfrequency, due to the fixed time delay per phase delay pad, thecomposite phase for each frequency hop provides a datapoint foranalyzing the various individual component delays within thetransponder. For a transponder with 16 potential delays, the responsemust be measured for at least 16 frequencies, in order to resolve theindividual delays. In practice, at least 32 frequencies, and morepreferably 128 frequencies are employed. Since a minimum of 16measurements must be made to analyze the simultaneous equationsrepresenting the delay element group arrangement of the transponder,processing may commence with partial data. However, a minimum set ofmeasurements may yield unreliable data, so a larger set is preferablyemployed. If the analyzer seeks to logically reorder the received signaldata into a chirp (linearly increasing frequency) waveform, then theanalysis must wait until the data is available for reordering.

The thus obtained sets of amplitudes at each interrogation frequencycontain the code or “signature” of the particular signal transformer ofthe interrogated transponder. This signature may be analyzed and decodedusing simultaneous equation solving techniques in known manner. Ofcourse, it is understood that the analyzed signals may be more complexthan D.C. voltages, and therefore the associated processing and analysissomewhat more sophisticated. However, in principal, the characterizationof the plurality of signals under differing excitation conditions isemployed in analogous fashion to resolve the encoded identification ofthe transponder.

In performing an analysis of the transponder signal, a number ofcompensations and corrections may be made. For example, the round tripsignal delay may be normalized, yielding an estimate of distance by atime of arrival technique. Likewise, any Doppler shift in the signal maybe determined and compensated, additionally allowing an indication ofrelative speed. These corrections may be implemented by altering thebaseband demodulation to compensate, or by predistorting theinterrogation wave as desired. These corrections, and others, may alsobe provided in a digital processing algorithm in the analyzer.

As noted above, there are a number of potential causes for variationsfrom the nominal delay periods of a transponder, including temperaturechanges, mask variations, manufacturing variations and randomvariations. Prior art chirp interrogation systems, which employedfrequency domain transformations, thus compensated for these factors byadjusting the boundaries of frequency bins. According to the presentinvention, these compensations are made in evaluating the simultaneousequations which represent the individual delays based on the sets ofreceived amplitudes of the demodulate signals. Since these techniquesare closely corresponding, the known techniques may be generally appliedto the delay data produced by the present invention. Thus, the actualindividual delays are determined based on the sets of equations, andthen interpreted based on the compensation factors.

It is therefore an object of the present invention to provide a methodfor analyzing a frequency hopping spread spectrum interrogated passiveacoustic transponder comprising the steps of receiving a transpondersignal, demodulating the transponder signal with a representation of theinterrogation signal, integrating the demodulated transponder signalfrom a frequency hop with an effective integration time constant smallerthan a duration of the frequency hop, determining a relativephase-amplitude response of the demodulated signal due to interrogationsignal perturbations within the transponder, based on the integrateddemodulated transponder signal, over a plurality of frequency hops, andanalyzing the determined phase-amplitude, response to determine a set ofcomponent delays within the transponder.

It is also an object of the present invention to provide a method foranalyzing a frequency hopping spread spectrum interrogated passiveacoustic transponder comprising the steps of exciting a transponder witha spread spectrum interrogation signal in a plurality of differingexcitation states, receiving modified signals from the transponder underthe plurality of differing excitation states, demodulating the receivedsignals, filtering the demodulated received signals with a widebandwidth filter, determining a characteristic perturbation of thedemodulated received signals for each excitation state, and analyzingthe determined characteristic perturbations to determine a set ofcomponent perturbation elements within the transponder.

It is a further object of the present invention to provide a method forinterrogating a passive acoustic transponder with a non-narrow bandfrequency interrogation signal having a plurality of differingexcitation states, receiving modified signals from the transponder underthe plurality of differing excitation states, demodulating the receivedsignals, filtering the demodulated received signals with a widebandwidth filter, having a time constant which is small with respect tothe period between transitions of excitation states, determining acharacteristic perturbation of the demodulated received signals for eachexcitation state, and analyzing the determined characteristicperturbations to determine a set of component perturbation elementswithin the transponder.

It is also an object according to the present invention to provide apassive radio frequency transponder system which employs an intermittentinterrogation signal.

It is a further object according to the present invention to provide apassive radio frequency transponder system for reading informationencoded into a passive device as a set of time-constants, comprisingexciting the passive device with a set of signals having differingfrequency components, receiving a modified set of signals from thepassive device with perturbations due to the time-constants, andfiltering the received signals with a filter having a time constantcomparable in magnitude to a maximum time constant of the passivedevice.

It is also an object of the invention to analyze a set of steady stateresponses, at differing frequencies, of an acoustic transponder.

It is a further object according to the present invention to provide apassive acoustic transponder interrogation system capable of compatiblyoperating with known tap-delay acoustic transponder devices, whileoperating with a spread spectrum interrogation signal, e.g., aspermissible under FCC regulations. The present invention also gains theknown advantages of spread spectrum communications systems.

It is another object of the invention to provide a system and method forinterrogating a passive acoustic transponder, producing a transpondersignal having characteristic set of signal perturbations in response toan interrogation signal, comprising a signal generator, producing aninterrogation signal having a plurality of differing frequencies; areceiver, for receiving the transponder signal; a mixer, for mixing thetransponder signal with a signal corresponding to the interrogationsignal, to produce a mixed output; an integrator, integrating the mixedoutput to define an integrated phase-amplitude response of the receivedtransponder signal; and an analyzer, receiving a plurality of integratedphase-amplitude responses corresponding to the plurality of differingfrequencies, for determining the characteristic set of signalperturbations of the passive acoustic transponder.

It is also an object of the invention to provide an apparatus and methodfor identifying a passive acoustic transponder or an object associatedtherewith, comprising placing a passive acoustic transponder inproximity to the object, the transponder having a set of characteristicsignal perturbations selected from a signal perturbation space having aplurality of degrees of freedom, and producing a perturbed signal inresponse to an interrogation; interrogating the passive acoustictransponder with a frequency hopping spread spectrum signal, having asequence of a plurality of different frequencies, and a dwell period;receiving and demodulating the perturbed signal based on arepresentation of the frequency hopping spread spectrum signal;determining an average phase-amplitude response of the demodulatedperturbed signal during a plurality of dwell periods; and analyzing theaverage phase-amplitude response from the plurality of dwell periods todetermine the values of the plurality of degrees of freedom. In thiscase, preferably, a plurality of passive acoustic transponders areprovided, with a database storing an association of each passiveacoustic transponder with values identifying the values of the pluralityof degrees of freedom and the identity of the associated object, withthe identity of the transponder or object retrieved from the databasebased on the determined values of the plurality of degrees of freedom.

The characteristic set of signal perturbations may include an acousticreflection pattern, a set of phase shifts, resonances, and/or amplitudeattenuation.

The interrogation signal preferably has a frequency band having abandwidth of less than about 5% and having a center frequency in therange of between about 300 MHz to about 30 GHz, and more preferably afrequency in a band between about 800 MHz and 1.3 GHz and having abandwidth of between about 1-3%. The interrogation signal may be, forexample, a frequency hopping spread spectrum signal, but may also have astepwise chirp or other waveform. Thus, the sequence of frequency hopsmay be random, pseudorandom (repeating sequence), or regular. Theinterrogation signal may produce the plurality of differing frequenciesindividually, or a plurality of frequencies concurrently. In the lattercase, typically a multiplexer or parallel processing path (a pluralityof mixers) would be necessary, each mixing a different frequencycomponent to produce a demodulated signal.

The characteristic set of signal perturbations includes a patternselected from a signal perturbation space having one or more degrees offreedom. Where the degrees of freedom include composite delays, thesemay be distinguished by providing an interrogation signal or set ofsignals including a plurality of differing frequencies no less in numberthan the number of degrees of freedom. There preferably are at least twotimes the number of differing frequencies as there are degrees offreedom, and, for example, 8 to 16 times the number of differingfrequencies in a set of discrete frequencies is a suitable range. Thediffering frequencies are preferably spread about evenly across a band,although the selection of frequencies may be optimized to distinguisheach degree of freedom, which may result in non-even spreading. Unevenlyspaced interrogation frequency embodiments may also be employed as knownin the art. The interrogation signal frequencies are preferablygenerated with a digitally controlled oscillator. In order to bettercorrelate the interrogation signal with the transponder signal, therepresentation of the interrogation signal fed to the mixer may bedelayed with respect to the interrogation signal.

The demodulator may also employ an independently generated signal.Therefore, if a phase shift or delay function between one portion andanother portion of the received signal includes the data to be acquired,then, assuming that the analyzer cannot deal directly with the modulatedsignal, then the mixer therefore must receive a signal from a localoscillator having a stable phase. Preferably, the received signal isdemodulated to or near baseband, although this is not required. Forexample, with a 915 MHz band spread spectrum signal with a bandwidth of20 MHz, the received signal may be downconverted to a 0-20 MHz bandspread spectrum signal and directly digitized, with subsequentprocessing being digital.

According to a preferred embodiment, the mixer serves as a homodynephase detector, mixing the transponder signal with a signalcorresponding to the interrogation signal to produce, in a steady statecondition, a signal whose change in amplitude corresponds to a relativephase difference and attenuation between the transponder signal and thesignal corresponding to the interrogation signal. The mixer ispreferably a double balanced mixer.

The integrator may be a low pass filter, preferably having at least twopoles in its transfer function, but may also be a complex structure,such as an active integrator over a predetermined timeperiod. Theintegrated phase-amplitude response is preferably represented as ascalar value for each differing frequency of the interrogation signal.As stated above, this integrator preferably has a wide bandwidth.

In one embodiment, the interrogation signal has a plurality ofsuccessive states, each state having a predetermined period, theintegrator comprising a low pass filter having a main time constant ofless than about 100% of the period between successive interrogationsignal states. For example, the characteristic set of signalperturbations of the transponder has a maximum significant time constantof less than about 5 μS and comprises a pattern selected from a signalperturbation space having about 16 degrees of freedom, the transponderintegrator interrogation signal being a pseudorandom sequence frequencyhopping signal having about 128 successive different frequencies beforerepetition, each state having a predetermined period of about 15 μS. Inthis case, the integrator time-constant is less than about 15 μS, andmore preferably about 8 μS.

In one embodiment, the interrogation signal comprises a frequencyhopping spread spectrum signal having a dwell period, the characteristicset of signal perturbations of the transponder having a maximumsignificant time constant, e.g., being less than about 50% of the dwellperiod, the integrator being a low pass filter having a reciprocal ofcutoff frequency of between about the maximum significant time constantof the transponder and the dwell period.

The transfer function of the integrator may be functionally described asfollows. The filter should settle to a desired degree, from the priornormal excitation condition and preferably even from a prior saturatedinput, before the first significant delayed signal returns from thetransponder, and should accurately pass the relative phase delay betweenthe transponder signal prior to the first significant encoding delay andsubsequent to the last significant encoding delay. Too long anintegration time will allow a form of intersymbol interference, and tooshort a time constant will be ineffective for antialiasing the signalfor digitization and otherwise allow excess noise into the analyzer.

The analyzer preferably evaluates a set of simultaneous equationsrelating the integrated phase-amplitude responses to the characteristicset of signal perturbations of the passive acoustic transponder. Thecharacteristic set of signal perturbations comprises a pattern selectedfrom a signal perturbation space having a plurality of degrees offreedom, the interrogation signal having a number of the plurality ofdiffering frequencies no less than the number of degrees of freedom, theanalyzer solving simultaneous equations for evaluating the degrees offreedom, the analyzer compensating the evaluated degrees of freedom forpredetermined variances, evaluating each integrated phase-amplituderesponse for consistency with a set of remaining integratedphase-amplitude responses, and outputting a compensated, self-consistentdata set corresponding to the evaluated degrees of freedom.

These and other objects will become apparent from a review of thedetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view, in enlarged scale, of a transponderconfiguration.

FIG. 2 is a plan view, in greatly enlarged scale, of a portion of thetransponder configuration shown in FIG. 1.

FIG. 3 is a diagram showing the respective time delays of the differentSAW pathways in the transponder of FIG. 1.

FIG. 4 is a flow diagram showing the order of calculations carried outby the signal processor.

FIG. 5 is a block diagram of a first embodiment of an acoustictransponder interrogation system according to the present invention.

FIG. 6 is a block diagram of a second embodiment of an acoustictransponder interrogation system according to the present invention,having a plurality of signal generators.

FIGS. 7A1, 7A2 and 7B are a schematic drawings of a single pole R-Cintegrator, a double pole R-C integrator and a switched integrator.

FIG. 8 is a flow chart showing the operation sequence of a systemaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedwith reference to the drawings. Identical elements in the variousfigures are designated with the same reference numerals.

An interrogation system according to the present invention is providedwhich employs a frequency hopping spread spectrum signal having apseudorandom sequence which excites a set of frequencies. Typically, theinterrogation sequence is repeated continuously. The interrogationsignal occupies, for example, a band of approximately 20 MHz centered at915 MHz. The band is divided into 128 discrete frequencies, each ofoccupies a period of about 15 μS. For example, with a frequency hoppingperiod of 15 μS, the system generates an interrogation pulse at aselected frequency for 7.5 μS, is silent and insensitive for 300 nS toeliminate near-field echos, and then listens for 4 μS for the returnsignal. The system is then silent and insensitive for the remainingportion of the cycle, about 3.5 μS.

The interrogation signal is generated by a digitally controlledoscillator, including a phase locked loop with voltage controlledamplifier. The sequence is generated by a sequence generator which setsthe digital controls of the digitally controlled oscillator to a desiredstate and with a desired sequence. One possible scheme is to evenlyspread energy through the band, without concentrating the wave energy ina narrow range for an extended period, with a repeated pseudorandomsequence of 128 different frequencies. The sequence generator mayinclude a lookup table or algorithmic pseudorandom sequence generator. Atypical passive transponder device for use with the system has 16degrees of freedom in its code space, generated by four bidirectionaltransducers, each wave having two sets of delay pad elements. Thus, theinterrogator system analyzer must resolve the 16 degrees of freedom inorder to identify the encoding of the transponder. In order to resolvethese degrees of freedom, at least 16 distinct excitation conditionsmust be applied to the transponder, producing a response which, whenanalyzed, allows solution of the simultaneous equations. Since at least16 conditions, in this case different frequencies, are required, thelarger available number, e.g., 128 frequencies, allows robustness tointerference and increased accuracy.

A microprocessor is provided to control the system, and, for example,generates the control signals for the digitally controlled oscillatorsignal generator and processes the output. It is understood that thevarious functions may be integrated into common circuits, such as analogapplication specific integrated circuits (ASICs), digital ASICs, and/ormixed signal ASICs.

Further, since only 16 discrete excitation parameters are required, ofthe 128 available, the analysis in the analyzer may proceed based on anincomplete data set. Because of thus flexibility, the frequency hoppingsequence from the sequence generator need not repeat or excite eachfrequency at the minimum rate, so long as the analyzer, to be describedlater, is provided with data identifying the set of excitationconditions, i.e., an information path from the sequence generator. Theanalyzer ultimately outputs identifying information for the transponder.

The receiver includes an antenna and amplifier, which receives themodified interrogation signal from the transponder. The modifiedinterrogation signal is mixed in a demodulator with a representation ofthe interrogation signal, which in this case is derived from theoscillator output during the receive period. The demodulator is a doublebalanced mixer, operating with inputs up to at least 1 GHz. As analternate to the oscillator signal, the representation of theinterrogation signal may a signal delayed by a delay element, orcomprise an independently generated signal which is coherent with theinterrogation signal. The purpose of this mixer is to translate thefrequency of the signal to baseband, but more importantly to allowhomodyne detection of the relative phase-amplitude relationship betweenthe interrogation signal and the transponder signal. Where the signalsare in phase and of the same frequency, the output of the mixer ismaximal, and decreases as the phases reach quadrature, turning negativeas the signals move completely out of phase. The amplitude of thereceived signal will also modify the phase-amplitude response from themixer. Due to the composite nature of the transponder signal, being thesuperposition of the modifications in each acoustic path in thetransponder device, as each component of the wave is initially receivedafter a frequency hop, the relative phase will change. After thetransient response has abated, the relative phase and amplitude will bestatic until the next hop. If the relative frequencies of thetransponder return signal and the interrogation signal are close, forexample having a relative phase shift during the integration period ofless than 10%, then the error due to this difference will be small.Therefore, in practice, a homodyne signal analysis may be applied in thepresence of a small difference in phase between the two signals.

It is noted that, as employed herein, the phrase “phase-amplitude”denotes the complex phase and amplitude characteristics of the signal,and therefore encompasses changes in relative phase, relative amplitude,and/or changes in relative phase and amplitude.

An integrator, which is a two pole R-C low pass filter, as shown in FIG.19A2, having two time-constants of about b 7 μS, and a frequency cutoffof about 150 kHz, receives the output of the mixer, and thus produces afiltered output representing the phase-amplitude response for eachexcitation frequency. The integrator output is sampled by a sample holdamplifier before and the transients due to the encoded delays within thetransponder.

Of course, the integrator may be more or less complex. It may be asingle pole R-C filter, as shown in FIG. 7A1, an active filter (notshown) or digitally controlled integrator having a controlledintegration period, as shown in FIG. 7B, or other type.

The duration of each hop of the signal generator is generally longerthan the longest delay in a transponder, as well as the travel delay.Thus, where a maximum delay within a transponder is less than about 10μS, a stationary frequency dwell period is greater than about 10 μS.

In the preferred embodiment, a single frequency is emitted 240 by signalgenerator 200, based on an input from the sequence generator 202, as theinterrogation signal at any time, which is transmitted to a transponder200, the modified signal from which is then received 242 by thereceiver, mixed 244 in mixer 208 with a representation 218 of theinterrogation signal, which is, for example, the signal from the signalgenerator 203, delayed by delay 208, integrated by integrator 210,analyzed 248 in analyzer 212, which outputs a set of characteristics214; however, a plurality of such frequencies may be emittedsimultaneously or concurrently, as shown in FIG. 6. The interrogationprocess includes producing a plurality of interrogation frequencies 246,the response to each of which is analyzed 248 and subjected to databaselookup 250, to determine the identification to be output 252. Thereceiver system may selectively decode one of the frequencies at anygiven time, or a parallel process established with a plurality of mixersand integrators. Thus, in the later case, a system as shown in FIG. 6 isprovided. A control 220 controls a pair of sequence generators 221, 222which in turn control a pair of signal generators 223, 224 which are,for example, digitally controlled oscillators. The outputs of the signalgenerators 223, 224 are summed and emitted from a transmitter 226, andinteract with a transponder 200. A receiver, 228 receives a modifiedinterrogation signal, which is then fed to a pair of mixers 230, 231 fordemodulation with signals corresponding to the individual signalcomponents of the interrogation signal. The outputs of the mixers 230,231 are individually integrated in integrators 232, 233 and the outputscaptured and analyzed in the analyzer 234. The analyzer 234, afteracquiring sufficient data and optionally performing consistency checks,outputs a set of characteristics 235 of the transponder. In comparisonto the system shown in FIG. 5, the system according to FIG. 6 willobtain sufficient data for an output about twice as fast. In likemanner, a greater number of channels may be simultaneously operative, upto the number of different frequencies.

The demodulator produces a resulting low frequency signal (nearbaseband), resulting from homodyne demodulation of the interrogationsignal with the transponder signal at the same frequency, thus producinga signal with a relative amplitude related to the average complexphase-amplitude relation of the signals entering the mixer. Because ofthe differences in the transponder signal due the fixed nature ofinternal delays and the changing interrogation frequency, the relativephase at each frequency hop provides a datapoint for analyzing thevarious delays within the transponder. The amplitude may also vary ininterrogation frequency-dependent manner due to the differences inconfiguration of each encoded transponder.

In performing an analysis of the transponder signal, a number ofcompensations and corrections may be made. For example, the round tripsignal delay may be normalized, yielding an estimate of distance by atime of arrival technique. Likewise, any Doppler shift in the signal maybe determined and compensated, allowing an indication of relative speed.This later correction produces a relative frequency shift of thetransponder signal with respect to the interrogation signal. Thisfrequency shift, however, is typically of a relatively low frequency,below the 66 kHz frequency hopping rate and therefore introduces onlysmall errors, which may be compensated in the analysis. Likewise, otherpotential causes for variations from the nominal delay periods of atransponder, including temperature changes, mask variations,manufacturing variations and random variations may also be compensatedin the analysis. Since the determined degrees of freedom correspond todelays, the correction scheme is essentially as shown in FIG. 16 of theprior art.

The relative phase-amplitude output from the integrator is digitized andstored in memory and analyzed under control of the microprocessor,preferably by a dedicated digital signal processor (DSP). This DSPdetermines the delay coefficients of the transponder, which correspondto the degrees of freedom, and applies corrections and compensations asnecessary. The DSP may also perform consistency checking of each datapoint, based on the redundant information from the larger number ofdatapoints available than are minimally necessary, excluding fromanalysis those which are likely to represent artifacts or interference.The microprocessor then receives the delay coefficients, which are usedto access a database, allowing identification of the transponder, whichis then output. Typically, the database also stores an association withan object, such as baggage, cargo, automobiles, or the like, which mayalso be accessed from the database.

The analyzer thus evaluates a set of simultaneous equations relating theintegrated phase-amplitude responses to the characteristic set of signalperturbations of the passive acoustic transponder, compensating theevaluated degrees of freedom for predetermined variances, evaluatingeach integrated phase-amplitude response for consistency with a set ofremaining integrated phase-amplitude responses, and producing an outputof the delay coefficients.

There has thus been shown and described a novel method for interrogatinga passive acoustic wave transponder with a frequency hoppinginterrogation wave, and a method and system for analyzing a transpondersignal therefrom. Many changes, modifications, variations and other usesand applications of the subject invention will, however, become apparentto those skilled in the art after considering this specification and theaccompanying drawings which disclose preferred embodiments thereof. Allsuch changes, modifications, variations and other uses and applicationswhich do not depart from the spirit and scope of the invention aredeemed to be covered by the invention which is limited only by theclaims which follow.

What is claimed is:
 1. A system for interrogating a passive acoustic transponder, producing a transponder signal having characteristic set of signal perturbations in response to an interrogation signal, comprising: (a) a signal generator, producing an interrogation signal having a plurality of differing frequencies; (b) a receiver, for receiving the transponder signal; (c) a mixer, for mixing said transponder signal with a signal corresponding to said interrogation signal, to produce a mixed output; (d) an integrator, integrating said mixed output to define an integrated phase-amplitude response of the received transponder signal for each of said differing frequencies; and (e) an analyzer, receiving a plurality of integrated phase-amplitude responses corresponding to said plurality of differing frequencies, for determining the characteristic set of signal perturbations of the passive acoustic transponder.
 2. The system according to claim 1, wherein the characteristic set of signal perturbations comprises an acoustic reflection pattern.
 3. The system according to claim 1, wherein the characteristic set of signal perturbations comprises a set of phase shifts.
 4. The system according to claim 1, wherein said interrogation signal comprises a frequency band having a bandwidth of less than about 5% and having a center frequency in the range of between about 300 MHz to about 30 GHz.
 5. The system according to claim 1, wherein said interrogation signal comprises a frequency in a band between about 800 MHz and 1.3 GHz and having a bandwidth of between about 1-3%.
 6. The system according to claim 1, wherein said interrogation signal comprises a frequency hopping spread spectrum signal.
 7. The system according to claim 1, wherein the characteristic set of signal perturbations comprises a pattern selected from a signal perturbation space having a plurality of degrees of freedom, said interrogation signal having a number of said plurality of differing frequencies no less than the number of degrees of freedom.
 8. The system according to claim 1, wherein said plurality of differing frequencies are generated sequentially.
 9. The system according to claim 1, wherein at least two of said plurality of differing frequencies are generated simultaneously.
 10. The system according to claim 1, wherein said interrogation signal has a pseudorandom sequence of differing frequencies which repeats after a finite duration.
 11. The system according to claim 1, wherein the characteristic set of signal perturbations comprises a pattern selected from a signal perturbation space having a plurality of degrees of freedom, said interrogation signal having a number of said plurality of differing frequencies between about 2 to 8 times the number of degrees of freedom.
 12. The system according to claim 1, wherein the characteristic set of signal perturbations comprises a pattern selected from a signal perturbation space having a plurality of degrees of freedom, said interrogation signal having a number of said plurality of differing frequencies at least 2 times the number of degrees of freedom.
 13. The system according to claim 1, wherein said plurality of differing frequencies are about evenly spaced across a band.
 14. The system according to claim 1, wherein said signal generator comprises a digitally controlled oscillator.
 15. The system according to claim 1, wherein said signal corresponding to said interrogation signal is delayed with respect to said interrogation signal.
 16. The system according to claim 1, wherein said mixer homodynes said transponder signal with a signal corresponding to said interrogation signal to produce, in a steady state condition, a signal whose amplitude corresponds to a relative phase-amplitude difference between said transponder signal and said signal corresponding to said interrogation signal.
 17. The system according to claim 1, wherein said mixer comprises a double balanced mixer.
 18. The system according to claim 1, wherein said integrator comprises a low pass filter.
 19. The system according to claim 1, wherein said integrator integrates said mixed output over a predetermined period.
 20. The system according to claim 1, wherein said integrated phase-amplitude response is represented as a scalar value.
 21. The system according to claim 1, wherein said integrator interrogation signal has a plurality of successive states, each state having a predetermined period, said integrator comprising a low pass filter having a main time constant of less than about 25% of said period.
 22. The system according to claim 1, wherein said characteristic set of signal perturbations of said transponder has a maximum significant time constant of less than about 5 μS and comprises a pattern selected from a signal perturbation space having about 16 degrees of freedom, said transponder integrator interrogation signal being a pseudorandom sequence frequency hopping signal having about 128 successive different frequencies before repetition, each state having a predetermined period of about 125 μS.
 23. The system according to claim 1, wherein said interrogation signal comprises a frequency hopping spread spectrum signal having a dwell period, the characteristic set of signal perturbations of said transponder having a maximum significant time constant of less than about 10% of said dwell period, said integrator being a low pass filter having a cutoff frequency of less than the reciprocal of the maximum significant time constant of the transponder.
 24. The system according to claim 1, wherein said integrator comprises a low pass filter having at least two poles in its transfer function.
 25. The system according to claim 1, wherein said analyzer evaluates a set of simultaneous equations relating said integrated phase-amplitude responses to the characteristic set of signal perturbations of the passive acoustic transponder.
 26. The system according to claim 1, wherein the characteristic set of signal perturbations comprises a pattern selected from a signal perturbation space having a plurality of degrees of freedom, said interrogation signal having a number of said plurality of differing frequencies no less than the number of degrees of freedom, said analyzer solving simultaneous equations for evaluating the degrees of freedom, said analyzer compensating said evaluated degrees of freedom for predetermined variances, evaluating each integrated phase-amplitude response for consistency with a set of remaining integrated phase-amplitude responses, and outputting a compensated, self-consistent data set corresponding to said evaluated degrees of freedom.
 27. The system according to claim 26, wherein said interrogation signal is produced intermittently.
 28. The system according to claim 1, wherein said signal generator produces said interrogation signal having a plurality of differing frequencies as a substantially complete, pseudorandomly ordered set of frequencies, which are evenly spaced through an interrogation frequency band.
 29. A method for interrogating a passive acoustic transponder, producing a transponder signal having characteristic set of signal perturbations in response to an interrogation signal, comprising: (a) producing an interrogation signal having a plurality of differing frequencies; (b) receiving the transponder signal from the passive acoustic transponder; (c) mixing the transponder signal with a signal corresponding to the interrogation signal, to produce a mixed output; (d) integrating the mixed output to define an integrated phase-amplitude response of the received transponder signal; and (e) analyzing a plurality of integrated phase-amplitude responses corresponding to the plurality of differing frequencies, to determine the characteristic set of signal perturbations of the passive acoustic transponder.
 30. A method for identifying a passive acoustic transponder, having a set of characteristic signal perturbations selected from a signal perturbation space having a plurality of degrees of freedom, and producing a perturbed signal in response to an interrogation, comprising the steps of: (a) interrogating the passive acoustic transponder with a frequency hopping spread spectrum signal, having a psueodorandom sequence of a plurality of different frequencies, and a stationary frequency dwell period; (b) receiving and demodulating the perturbed signal based on a representation of the frequency hopping spread spectrum signal; (c) determining an average phase-amplitude response of the demodulated perturbed signal during a plurality of dwell periods; and (d) analyzing the average phase-amplitude response from the plurality of dwell periods to determine the values of the plurality of degrees of freedom.
 31. The method according to claim 30, further comprising the steps of: (e) providing a plurality of passive acoustic transponders; (f) storing in a database an association of an identification of each passive acoustic transponder with values identifying the values of the plurality of degrees of freedom; and (g) based on the determined values of the plurality of degrees of freedom, retrieving an identification of a passive acoustic transponder from the database.
 32. The method according to claim 30, further comprising the steps of providing a plurality of passive acoustic transponders, each physically associated with an object; storing in a database an association of each passive acoustic transponder with the object, including the values identifying the values of the plurality of degrees of freedom; and based on the determined values of the plurality of degrees of freedom, retrieving an identification of an object associated with the transponder from the database.
 33. The method according to claim 30, wherein said frequency hopping spread spectrum signal further comprises a quiet period during which no signal is emitted.
 34. A system for determining characteristics of an acoustic wave transponder, producing a transponder signal having characteristic set of signal perturbations in response to an interrogation signal and an internal reference, comprising: (a) an interrogation signal generator, producing a non-stationary interrogation signal hopping to a plurality of differing frequencies; (b) a receiver, for receiving the transponder signal; (c) a demodulator, for demodulating a signal dependent on characteristics of the transponder from said interrogation signal, to produce a demodulated output; (d) a phase-amplitude detector, detecting a phase-amplitude relationship of said demodulated output with respect to the reference; and (e) an analyzer, sequentially receiving a plurality of detected phase-amplitude relationships corresponding to said plurality of differing frequencies, for determining the characteristic set of signal perturbations of the acoustic wave transponder.
 35. The system according to claim 34, wherein said non-stationary interrogation signal hops to a plurality of differing frequencies as a substantially complete, pseudorandomly ordered set of frequencies, which are evenly spaced through an interrogation frequency band.
 36. The system according to claim 34, wherein said interrogation signal generator produces an intermittent signal.
 37. A method for analyzing a frequency hopping spread spectrum interrogated passive acoustic transponder comprising the steps of: receiving a transponder signal; demodulating the transponder signal with a representation of the interrogation signal; integrating the demodulated transponder signal from a frequency hop with an effective integration time constant smaller than a duration of the respective frequency hop; determining a relative phase-amplitude shift of the demodulated signal due to interrogation signal perturbations within the transponder, based on the integrated demodulated transponder signal, over a plurality of frequency hops; and analyzing the determined phase-amplitude shifts to determine a set of component delays within the transponder.
 38. A method for analyzing a frequency hopping spread spectrum interrogated passive acoustic transponder comprising the steps of: exciting a transponder with a spread spectrum interrogation signal in a plurality of differing excitation states; receiving modified signals from the transponder under the plurality of differing excitation states; demodulating the received signals; filtering the demodulated received signals with a wide bandwidth filter; determining a characteristic perturbation of the demodulated received signals for each differing excitation state; and analyzing the determined characteristic perturbations to determine a set of component perturbation elements within the transponder.
 39. A method for interrogating a passive acoustic transponder with a non-narrow band frequency interrogation signal having a plurality of differing excitation states, comprising the steps of: receiving modified signals from the transponder under the plurality of differing excitation states; demodulating the received signals; filtering the demodulated received signals with a wide bandwidth filter, having a time constant which is small with respect to the period between transitions of differing excitation states; determining a characteristic perturbation of the demodulated received signals for each differing excitation state; and analyzing the determined characteristic perturbations to determine a set of component perturbation elements within the transponder.
 40. A passive radio frequency transponder method for reading information encoded into a passive device, having a maximum time constant, as a set of phase-amplitude variations of signals having respective variation time constants shorter than said maximum time constant, comprising the steps of: exciting the passive device with a set of signals having differing frequency components; receiving a modified set of signals from the passive device with phase-amplitude variations; and filtering the received signals with a filter having a time constant comparable in magnitude to the maximum time constant of the passive device.
 41. A passive acoustic transponder interrogation system, comprising: a transponder, producing a transponder signal having characteristic set of signal perturbations in response to an interrogation signal; a signal generator, producing an interrogation signal having a plurality of differing frequencies; a receiver, for receiving the transponder signal; a mixer, for mixing the transponder signal with a signal corresponding to the interrogation signal, to produce a mixed output; an integrator, integrating the mixed output to define an integrated phase-amplitude response of the received transponder signal; and an analyzer, receiving a plurality of integrated phase-amplitude responses corresponding to the plurality of differing frequencies, for determining the characteristic set of signal perturbations of the passive acoustic transponder.
 42. A method for identifying a passive acoustic transponder or an object associated therewith, comprising: placing a passive acoustic transponder in proximity to the object, the transponder having a set of characteristic signal perturbations selected from a signal perturbation space having a plurality of degrees of freedom, and producing a perturbed signal in response to an interrogation; interrogating the passive acoustic transponder with a pseudorandom order, frequency hopping spread spectrum signal, having a sequence of a plurality of different frequencies, and a dwell period; receiving and demodulating the perturbed signal based on a representation of the pseudorandom order, frequency hopping spread spectrum signal; determining an average phase-amplitude response of the demodulated perturbed signal during a plurality of dwell periods; and analyzing the average phase-amplitude response from the plurality of dwell periods to determine the values of the plurality of degrees of freedom. 